This invention relates to the inspection of components by self generated transient stress wave detection.
If certain components are tapped or otherwise mechanically excited in order to cause them to vibrate, they generate acoustic emissions which are peculiar to the components concerned. Variations in the acoustic emissions of components which are generally similar do arise however if a component has a fault, such as a crack, within it. This being so, it is possible to identify faulty components by determining their acoustic emissions when excited and comparing those acoustic emissions with the acoustic emissions from a component which is known not to be faulty.
A drawback with this technique is that although it is effective in detecting faults in components, it is difficult to determine their location and extent. Usually other techniques such as X-ray photography or ultrasonic inspection have to be employed in order to obtain this information. X-ray photography carries potential health hazards and both X-ray photography and ultrasonic inspection are difficult to use on certain components which have to be inspected whilst they are located on other components.
A further drawback is that the means which is used to mechanically excite the component may itself generate acoustic emissions which could mask the acoustic emissions which are generated within the component. Such acoustic emissions could for instance be generated by fretting between the component and the exciter means.
One example of a component, the inspection of which introduces these difficulties, is a ducted fan gas turbine engine fan blade which is either of laminated construction or is made up of a metal skin surrounding a core of a suitable lightweight material. In both modes of construction, the fan blades have to be inspected in order to detect any bonding effects between the laminates or between the core and the skin. Since these defects can arise during engine operation, it is necessary to have some form of inspection technique which can be employed whilst the fan blades are located on the engine. X-ray photography and ultrasonic inspection are not easily amenable to this form of inspection.
It is an object of the present invention to provide a method of inspecting a component for defects which method may be used in locations in which the use of alternative methods of inspection is inconvenient or even impossible, and which, in certain embodiments, is capable of determining the location of those defects.